Electrode supports for arc lamps



Feb. 10, 1970 J. F. RICHTER 3,495,118

ELECTRODE SUPPORTS FOR ARC LAMPS Filed March 4, 1968 3 20 I2 0 F IG.IA 22 (E 52 L I 30 f INVENTOR.

JOHN F. RICHTER United States Patent 3,495,118 ELECTRODE SUPPORTS FOR ARC LAMPS John F. Richter, San Francisco, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Mar. 4, 1968, Ser. No. 710,280 Int. Cl. H01 5/16; H01k 1/28 US. Cl. 313112 11 Claims ABSTRACT OF THE DISCLOSURE An arc lamp having a sealed envelope housing two axially-aligned electrodes spaced from each other a distance defining a light-generating arc gap. The envelope comprises a metallic tubular member and an optical window through which the light passes as a beam parallel to the electrodes. A plurality of struts are provided for radially supporting at least one of the electrodes. One end of the strut is affixed to the tubular member and the other end to the electrode. The strut may be joined tangentially to the electrode to translate expansive forces of the strut from normal to rotational forces on the electrode when the arc lamp is operated. The struts may also be arcuate to relieve thermal stress therein through an increase in the curvature of the strut arc.

BACKGROUND OF THE INVENTION This invention relates generally to electrode supports for are lamps, and particularly to electrode supports for sealed beam, high intensity short are lamps having electrodes mounted axially the lamp beam such as that disclosed in co-pending application Ser. No. 655,717 filed Aug. 14, 1967 by the same inventor.

Sealed beam, high intensity short arc lamps typically contain two axially-aligned electrodes spaced apart a short distance from each other to form an arc gap which is located in a vacuum-tight envelope. The arc gap typically measures some 4 to mm. in length. An ionizable gas is confined within the lamp envelope under pressure. The envelope usually comprises an opaque, tubular section, an arcuate reflector brazed to one end of the tubular section, and a planar optical window brazed to the other end of the tubular section. In some designs a second window is substituted for the reflector.

The two electrodes may be axially aligned within the envelope parallel to the optical window and radial to the lamp beam. In such configuration the electrodes will typically penetrate the tubular section of the envelope where they are accessible to electrical power. However, it has been found that several advantages over this configuration may be obtained by mounting the two electrodes normal to the planar window and axially the lamp beam. One such advantage is an increase in optical efiiciency of the lamp occasioned by allowing efficient flux collection and narrow beam shaping. This results in part by a lessening of electrode shadowing where the electrodes are aligned axial rather than transverse to the beam, and by avoidance of arc absorption of reflected radiation in its own wavelength. Though the nonuniformity in the beam caused by such shadowing and are absorption may be mitigated by the use of specially designed reflectors, the need for such costly components does not exist where the electrodes are axially mounted parallel to the lamp beam.

The configuration of electrodes mounted axially the lamp beam is not without attendant problems, however. One problem created by such arrangement is the manner in which the electrode located adjacent the optical window is supported. Linear extension of the electrode through the window is not practical due to the resulting need for a metal-to-glass or ceramic seal. Such seals are ICC inherently difficult to achieve, expensive, and have a significant unreliability factor. In high intensity are lamps the electrodes are heated to extremely high temperatures making a seal between the metal electrode and a ceramic or glass material very difficult to achieve since such materials have different coefficients of thermal expansion. Then too, the internal atmosphere of the lamp may reach a pressure of some l050 times that of the atmosphere external to the lamp. This of course would place an added :burden on the seal. Since the electrode adjacent the optical window can be mounted to a metallic portion of the envelope or joined to an opaque, ceramic envelope section at a ceramic-to-metal envelope seal, it is logical to choose such over that of mounting the electrode through the lamp window.

Having seen the advantages of mounting electrodes adjacent optical windows to an envelope section other than that of the window itself, one has coincidentally decided on an electrode support orientation having at least a substantial component normal to the electrode itself. This is true due to the fact that the optical window must occupy most if not all of the projection end of the lamp for optimum beam efliciency. By preempting this space, little remains at the beam emission end of the lamp for a metallic portion of the envelope to occupy to which an electrode support might be mounted. An area of such metallic portion in abutment with the window and its seal would also be a poor choice for support mounting due to the high temperatures to which the electrode and its support are subjected during lamp operation.

Thus we see that a support for electrodes situated axially the lamp beam should preferably be disposed normal, or having a normal component, to the electrode itself. As the lamp beam passes through at least most of the space within the lamp envelope about the electrode, the support will necessarily be located Within the beam. Obviously, the support should block as little of the beam as possible and thus should be made as small as is practical in the plane normal to the electrode. This raises the question of how many supports should be used.

One support is not feasible because such would expand when the lamp is heated thereby displacing the electrode in its spacial relationship with the other electrode, the reflector, the lamp focal point and the lamp beam. Mounting the electrode with allowance for such displacement predesigned into the lamp is also not feasible due to imprecision with which such displacement can be predicated in practice under varying lamp orientations and environmental conditions. Accordingly, two or more electrode supports would appear necessary. Such plurality would provide more rigid support and would enable the thermal expansion of each support to be counterbalanced by that of the other suppo ts. Though such is theoretically possible, in practice it has been found that an exact counterbalancing is extremely difiicult to achieve when the high intensity lamp is operated and the electrodes are subjected to extremely high temperatures.

Accordingly, it is a general object of the present invention to provide improved support for are lamp electrodes.

More specific, an object of the invention is to provide improved supports for sealed beam, high intensity are lamp electrodes which are oriented axially the lamp beam.

Another object of the invention is to provide means for maintaining high intensity are lamp electrodes in a fixed position relative to each other and to the focal point of an adjacent reflector during widely variant thermal conditions.

Yet another object of the invention is to provide electrode supports for high intensity are lamps which translate, under elevated thermal conditions, only a portion of their expansive force to the electrode thereby lessening 3 the force such supports exert on the electrodes during lamp operation.

Another object of the invention is to provide rugged electrode supports for arc lamp capable of withstanding substantial shock and vibration.

SUMMARY OF THE INVENTION Briefly described, the present invention resides in an arc lamp having a sealed envelope housing two axially aligned electrodes spaced from each other a distance defining a light generating arc gap. The sealed envelope comprises a metallic tubular member and an optical window through which the light passes as a beam parallel to the aligned electrodes. A plurality of supports are provided for at least one of the electrodes, each of the supports comprising a metallic strut. One end of the strut is affixed to the tubular member and the other end is affixed to the electrode. The strut may be joined tangentially to the electrode to translate expansive forces of the strut from normal to rotational forces on the electrode when the arc lamp is operated and the struts are heated. Furthermore, the struts may be arcuate in which case some of the thermal stress of the struts is relieved through an increase in the curvature of the strut are thereby transmitting less expansive force of the strut to the supported electrode.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1A is a cross-sectional view of a sealed beam, high intensity short are lamp having one embodiment of the present invention. FIGURE 1B is a frontal view of the lamp shown in FIGURE 1A.

FIGURE 2 is a frontal view of an arc lamp having another embodiment of the present invention.

FIGURE 3 is a frontal vieW of an arc lamp having another embodiment of the present invention.

FIGURE 4 is a frontal view of an arc lamp having yet another embodiment of the present invention.

FIGURE 5 is a frontal view of an arc lamp having yet another embodiment of the present invention.

FIGURE 6 is a frontal view of an arc lamp having yet another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in more detail to the drawing, there is illustrated in FIGURE 1A a sealed beam, high intensity are lamp in which the electrodes are oriented axially the lamp beam.

The lamp envelope comprises a ceramic cylinder section 10 which preferably is made of polycrystalline alumina. One end of this section is brazed to a ductile metallic ring 12 which in turn is brazed to a metallic tubular member 44 which may be spherical, ellipsoidal or parabolic. The ductile metallic ring serves as a stressrelieving portion of the envelope. The inner surface of member 44 serves as integral reflector 46. The other end of ceramic member 10 is brazed to a ductile metallic ring 18 which in turn is brazed to one side of a rigid metallic terminal ring 20. The terminal ring is then brazed to another ductile metallic ring 22 which in turn is brazed to the flange of a tubular, rigid metallic window support 24. As in the case of ring 12, the ductile metallic rings 18 and 22 serve to relieve stresses. The periphery of a disc-shaped window 26, which may be sapphire, is slightly recessed within and brazed to window support 24.

A cylindrical, metallic anode 28, which for example may be tungsten, is supported along the axis of tubular ceramic member 10 and window 26 by three trapezoidal, metallic struts 30 which may be made of molybdenum, for example. A trapezoidal rather than rectangular shape is used to insure an electrical stand-off between member 44 which is at the potential of the lamp cathode. Each strut has a notch into which terminal ring 20 is brazed.

The struts provide electrically conductive paths between anode 28 and terminal ring 20.

A cylindrical, metallic cathode 32 which may be made of thoriated tungsten, for example, is supported adjacent anode 28 and the axis thereof by a metallic cup 34. This cup, which forms a portion of the sealed envelope, is brazed about the periphery of an aperture in ellipsoidal member 44. A copper exhaust tubulation 36 communicates through the cup into the interior region of the envelope. The envelope is filled with an ionizable gas, such as xenon, under pressure. Exhaust tubulation 36 is then pinched off, thereby confining the pressurized gas Within the sealed envelope.

FIGURES 2-6 illustrate alternative configurations which the electrode and its supporting struts may take.

In FIGURE 2, a cylindrical electrode 40 is supported by arcuate struts 42, but in this configuration, as opposed to that shown in FIGURE 13, the inside edge of the arcuate surface of the struts, are brazed to the electrode. Both embodiments of FIGURES 1 and 2 have, however, two common features which provide stability of support during thermal cycling: Each strut is arcuate, and each is tangentially aflixed to the supported electrode. As the electrode and struts heat during initial lamp operation, they naturally expand. Should an end of the struts be joined normally to the electrode surface as in the case of electrode 48 and struts 50 in FIGURE 3, the thermal expansive force will be structurally resisted. Should such expansive force in each strut be identical, the electrode would retain its disposition relative to other lamp components. Nevertheless, each strut will undergo significant compression, thereby lessening its ruggedness. Furthermore, in practice each strut will not be subjected to identical degree of strain. This is due in part to the fact that the upper portion of the electrode and struts located thereabove become hotter than corresponding lower portions due to thermal convection. This is also due to the fact that the assembly will rarely have perfect symmetry. As a result, one strut will exert greater-expansive force on the electrode than another. This in turn will cause electrode 48 to change its position radially and thus its position relative to the adjacent electrode and the focal point of the reflector. Such deviation from the predesignated location of the electrode adversely affects the energy conversion and collection efliciency of the lamp. This problem is alleviated in the embodiment shown in FIGURES 1 and 2. As the arcuate struts here are subjected to internal expansive force, such force is relieved by an increase in the curvature of the arcuate strut. This release results in a substantial reduction in the expansive force which is translated to compressive force, thereby maintaining support ruggedness. This reduction also lessens the strut expansive force which is exerted on the electrode itself. This, of course, reduces any deviation of the electrode from its designed location caused by variations in the expansive force exerted by individual struts. In addition, the fact that each strut is aflixed tangentially to the electrode causes the reduced residual expansive force of each strut to impart torque to the electrodes in FIGURES 1B and 2 rather than radial force as do struts 50 in FIGURE 3. Uniform rotation of the electrode does not result in a deviation in its position relative to the adjacent electrode and reflector. Though the rotation may not be exactly uniform in practice, as a result of deviation in the reduced expansive force exerted by each strut on the electrode, such nonuniformity and radial disclocation is relatively minor in comparison with the radial dislocation which would be caused by the same deviation from struts mounted normal to the electrode surface.

The embodiment shown in FIGURE 3, in which three arcuate struts 5-0 are brazed normal to the surface of cylindrical electrode 48, offers the advantage over the embodiments shown in FIGURES 1 and 2 of ease of assembly in brazing the end of the struts to the electrode. Of

course the benefit of tangential mounting aforementioned is here sacrificed.

FIGURE 4 illustrates another combination of previously described features in which three rectilinear struts 54 are tangentially affixed to a cylindrical electrode 52. Here again a balance is reached between the selection of less costly components and their assembly, and the accompanying sacrifice of the aforementioned benefits which are gained from the use of arcuate struts.

The electrode need not be cylindrical as described above. FIGURE 5, for example, shows an arc lamp having an elongated electrode 56 mounted axially the lamp beam in which case the-electrode has a hexagonal crosssection. Three rectilinear struts 58 are brazed to planar surfaces of the hexagonal electrode. This structure is easy to assemble and quite rugged. In FIGURE 6 another hexagonal electrode 60 is supported by arcuate struts 62 which are brazed to planar surfaces of the electrode. This embodiment offers the aforementioned advantages gained from the arcuate design, the tangential mounting and ease of brazing planar surfaces.

It should be understood that the above-described embodiments are merely illustrative of application of the principles of the invention. Obviously, many modifications and combinations may be made in the illustrated examples without departing from the spirit and scope of the invention as set forth in the concluding claims. For instance, though three struts are preferred since this is the minimum number which provide sound support in every radial direction about the electrode, other quantities may be used. Likewise, the struts themselves may assume many possible configurations in employing the principles of the invention, as may the axially aligned electrode.

What is claimed is:

1. An arc lamp comprising a sealed envelope, two axially aligned electrodes housed within said envelope and spaced from each other a distance defining a light-generating arc gap, said sealed envelope having a metallic tubular member and an optical window through which said light passes as a beam parallel said axially aligned electrodes, and a plurality of supports for at least one of said electrodes, each of said supports comprising a metallic strut, one end of said strut being affixed to said tubular member and the other end of said strut being affixed to said one electrode, said plurality of supports being dimensioned, disposed within said envelope, and affixed to said one electrode in such manner as to translate at least a portion of the thermal expansive and contractive forces exerted by said supports on said one electrode during thermal cycling of the arc lamp from radial to rotational force on said one electrode whereby said one electrode maintains a substantially fixed position with respect to the other electrode during said thermal cycling.

2. An arc lamp in accordance with claim 1 wherein said strut is tangentially afiixed to said one electrode.

3. An arc lamp in accordance with claim 1 wherein said one electrode is cylindrical.

4. An arc lamp in accordance with claim 1 wherein said one electrode has a plurality of planar surfaces to which said plurality of supports are respectively afiixed.

5. An arc lamp in accordance with claim 1 wherein said strut is trapezoidal.

6. An arc lamp in accordance with claim 1 wherein said strut is arcuate.

7.- An arc lamp in accordance with claim 6 wherein said strut is tangentially affixed to said one electrode.

8. An arc lamp in accordance with claim 6 wherein said arcuate strut has an outer curved surface and an inner curved surface, and wherein a portion of said outer curved surface abuts said one electrode.

9. An arc lamp in accordance with claim 6 wherein said arcuate strut has an outer curved surface and an inner curved surface, and wherein a portion of said inner curved surface abuts said one electrode.

10. An arc lamp in accordance with claim 1 wherein said one electrode is a right-circular cylinder and said plurality of supports comprises three metallic struts brazed to the surface of said cylinder at points approximately equal distance from each other.

11. An arc lamp in accordance with claim 6 wherein said strut is radially afiixed to said one electrode.

References Cited UNITED STATES PATENTS 3,250,941 5/1966 Wilson et al. 313244 RAYMOND F. HOSS-FELD, Primary Examiner US. Cl. X.R. 

